1. Field of the Invention
The present invention relates to the structure of a center pin assembly inserted into the winding center of a winding-type electrode assembly of an electrochemical device, which has a case containing the winding-type electrode assembly, and more particularly to a center pin assembly adapted to secure the safety of an electrochemical device when physical impact (e.g. squeezing, shock) is applied from the outside, when the internal temperature rises, and/or when the device is overcharged.
2. Description of the Prior Art
As generally known in the art, lithium secondary batteries with high energy density have been extensively developed in line with the recent trend towards compact and light electronic equipment, as well as the widespread use of portable electronic devices.
Lithium secondary batteries use substances that are adapted to intercalate and deintercalate lithium ions as their cathode and anode, respectively, and an organic electrolyte or a polymer electrolyte fills the space between the cathode and anode. The Lithium secondary batteries create electric energy based on the oxidation and reduction reactions occurring when lithium ions are intercalated to and deintercalted from the cathode and anode, respectively.
However, lithium secondary batteries have a safety problem, and various efforts have been made to solve it.
If a lithium secondary battery is overcharged, an excessive amount of lithium is deintercalated from the cathode and is intercalated to the anode. As a result, lithium metal with very high reactivity precipitates on the anode surface. The cathode also becomes thermally unstable. The organic solvent used as the electrolyte undergoes a decomposition reaction, and the resulting abrupt exothermic reaction causes the battery to catch fire or explode.
In addition, lithium secondary batteries using a flammable non-aqueous electrolyte generally explode or catch fire due to a flammable gas resulting from the decomposition reaction of the electrolyte when the internal temperature of the batteries rises, due to a flammable gas resulting from the reaction between the electrolyte and the electrodes, or due to oxygen created as a result of decomposition of the cathode.
Furthermore, if a battery is crushed by a heavy object, if strong impact acts on the battery, or if the battery is exposed to a high temperature, the safety problem occurs. Particularly, the cathode and anode inside the battery are locally short-circuited if a nail penetrates the battery, if the battery is squeezed, if impact is applied to the battery, or if the battery is exposed to a high temperature. As a result, an excessive amount of current locally flows and generates heat. The amount of short-circuit current resulting from the local short circuit is inversely proportional to the resistance, and a larger amount of short-circuit current flows toward portions with low resistance. In this case, a very large amount of heat is locally generated around the short-circuited portion.
If heat is generated inside a battery, the cathode, anode, and electrolyte, which constitute the interior of the battery, react with one another or burn. This reaction is highly exothermic, and the battery finally catches fire or explodes. Therefore, such abrupt heat generation inside the battery must be avoided.
In an attempt to improve the safety in the case of overcharging, an additive is added to the non-aqueous electrolyte. However, the safety cannot be secured simply by adding an additive to the non-aqueous electrolyte when a nail penetrates the battery, when the battery is squeezed, when an impact acts on the battery, or when the battery is exposed to a high temperature, as mentioned above.